Novel structures for alpha-actinin:F-actin interactions and their implications for actin-membrane attachment and tension sensing in the cytoskeleton

We have applied correspondence analysis to electron micrographs of 2-D rafts of F-actin cross-linked with alpha-actinin on a lipid monolayer to investigate alpha-actinin:F-actin binding and cross-linking. More than 8000 actin crossover repeats, each with one to five alpha-actinin molecules bound, were selected, aligned, and grouped to produce class averages of alpha-actinin cross-links with approximately 9-fold improvement in the stochastic signal-to-noise ratio. Measurements and comparative molecular models show variation in the distance separating actin-binding domains and the angle of the alpha-actinin cross-links. Rafts of F-actin and alpha-actinin formed predominantly polar 2-D arrays of actin filaments, with occasional insertion of filaments of opposite polarity. Unique to this study are the numbers of alpha-actinin molecules bound to successive crossovers on the same actin filament. These "monofilament"-bound alpha-actinin molecules may reflect a new mode of interaction for alpha-actinin, particularly in protein-dense actin-membrane attachments in focal adhesions. These results suggest that alpha-actinin is not simply a rigid spacer between actin filaments, but rather a flexible cross-linking, scaffolding, and anchoring protein. We suggest these properties of alpha-actinin may contribute to tension sensing in actin bundles.

Ca2+ sensitivity of regulated cardiac thin filament sliding does not depend on myosin isoform

Myosin heavy chain (MHC) isoforms in vertebrate striated muscles are distinguished functionally by differences in chemomechanical kinetics. These kinetic differences may influence the cross-bridge-dependent co-operativity of thin filament Ca(2+) activation. To determine whether Ca(2+) sensitivity of unloaded thin filament sliding depends upon MHC isoform kinetics, we performed in vitro motility assays with rabbit skeletal heavy meromyosin (rsHMM) or porcine cardiac myosin (pcMyosin). Regulated thin filaments were reconstituted with recombinant human cardiac troponin (rhcTn) and alpha-tropomyosin (rhcTm) expressed in Escherichia coli. All three subunits of rhcTn were coexpressed as a functional complex using a novel construct with a glutathione S-transferase (GST) affinity tag at the N-terminus of human cardiac troponin T (hcTnT) and an intervening tobacco etch virus (TEV) protease site that allows purification of rhcTn without denaturation, and removal of the GST tag without proteolysis of rhcTn subunits. Use of this highly purified rhcTn in our motility studies resulted in a clear definition of the regulated motility profile for both fast and slow MHC isoforms. Maximum sliding speed (pCa 5) of regulated thin filaments was roughly fivefold faster with rsHMM compared with pcMyosin, although speed was increased by 1.6- to 1.9-fold for regulated over unregulated actin with both MHC isoforms. The Ca(2+) sensitivity of regulated thin filament sliding speed was unaffected by MHC isoform. Our motility results suggest that the cellular changes in isoform expression that result in regulation of myosin kinetics can occur independently of changes that influence thin filament Ca(2+) sensitivity.

Three-dimensional structure of the myosin V inhibited state by cryoelectron tomography

Unconventional myosin V (myoV) is an actin-based molecular motor that has a key function in organelle and mRNA transport, as well as in membrane trafficking. MyoV was the first member of the myosin superfamily shown to be processive, meaning that a single motor protein can 'walk' hand-over-hand along an actin filament for many steps before detaching. Full-length myoV has a low actin-activated MgATPase activity at low [Ca2+], whereas expressed constructs lacking the cargo-binding domain have a high activity regardless of [Ca2+] (refs 5-7). Hydrodynamic data and electron micrographs indicate that the active state is extended, whereas the inactive state is compact. Here we show the first three-dimensional structure of the myoV inactive state. Each myoV molecule consists of two heads that contain an amino-terminal motor domain followed by a lever arm that binds six calmodulins. The heads are followed by a coiled-coil dimerization domain (S2) and a carboxy-terminal globular cargo-binding domain. In the inactive structure, bending of myoV at the head-S2 junction places the cargo-binding domain near the motor domain's ATP-binding pocket, indicating that ATPase inhibition might occur through decreased rates of nucleotide exchange. The actin-binding interfaces are unobstructed, and the lever arm is oriented in a position typical of strong actin-binding states. This structure indicates that motor recycling after cargo delivery might occur through transport on actively treadmilling actin filaments rather than by diffusion.

Structure of the alpha-actinin-vinculin head domain complex determined by cryo-electron microscopy

The vinculin binding site on alpha-actinin was determined by cryo-electron microscopy of 2D arrays formed on phospholipid monolayers doped with a nickel chelating lipid. Chicken smooth muscle alpha-actinin was cocrystallized with the beta1-integrin cytoplasmic domain and a vinculin fragment containing residues 1-258 (vinculin(D1)). Vinculin(D1) was located at a single site on alpha-actinin with 60-70% occupancy. In these arrays, alpha-actinin lacks molecular 2-fold symmetry and the two ends of the molecule, which contain the calmodulin-like and actin binding domains, are held in distinctly different environments. The vinculin(D1) difference density has a shape very suggestive of the atomic structure. The atomic model of the complex juxtaposes the alpha-actinin binding site on vinculin(D1) with the N-terminal lobe of the calmodulin-like domain on alpha-actinin. The results show that the interaction between two species with weak affinity can be visualized in a membrane-like environment.

Blood-feeding in the young adult filarial worms Litomosoides sigmodontis

In this study with the filarial model Litomosoides sigmodontis, we demonstrate that the worms ingest host red blood cells at a precise moment of their life-cycle, immediately after the fourth moult. The red blood cells (RBC) were identified microscopically in live worms immobilized in PBS at 4 degrees C, and their density assessed. Two hosts were used: Mongolian gerbils, where microfilaraemia is high, and susceptible BALB/c mice with lower microfilaraemia. Gerbils were studied at 12 time-points, between day 9 post-inoculation (the worms were young 4th stage larvae) and day 330 p.i. (worms were old adults). Only the very young adult filarial worms had red blood cells in their gut. Haematophagy was observed between days 25 and 56 p.i. and peaked between day 28 and day 30 p.i. in female worms. In males, haematophagy was less frequent and intense. Similar kinetics of haematophagy were found in BALB/c mice, but frequency and intensity tended to be lower. Haematophagy seems useful to optimize adult maturation. These observations suggest that haematophagy is an important step in the life-cycle of L. sigmodontis. This hitherto undescribed phenomenon might be characteristic of other filarial species including human parasites.

The pathogenesis of chorioretinal disease in onchocerciasis

The pathogenesis of onchocercal chorioretinopathy is poorly understood. In this article, Philip Cooper, Ronald Guderian, Roberto Proaño and David Taylor discuss the important clinical, histological and epidemiological features of the resulting lesions that cause blindness, and review the numerous mechanisins that have been put forward to explain its pathogenesis. The pathogenesis of anterior segment disease, particulary sclerosing keratitis, has been reviewed in depth previously(1) and will not be discussed here.

Understanding Canadian student motivations and beliefs about giving blood

BACKGROUND

A better understanding of Canadian blood donor beliefs and motivations is needed to develop targeted interventions. Recruiters must know how motivation variables and correlation patterns differ with donor experience and sex.

STUDY DESIGN AND METHODS

Data addressing reasons for donating, statements about the blood supply, beliefs about donation consequences, and reasons for avoiding donation were collected from 450 undergraduates. Principal components analysis was used to investigate the underlying factorial structure of each domain. Men-women and donor-nondonor differences were explored with multivariate analysis of variance techniques.

RESULTS

A bivariate model better represented donor beliefs than did a bipolar conceptualization. Negative beliefs distinguished donors and nondonors more so than did positive factors. Altruism dominated reasons for donating, whereas logistic factors accounted for the most variance in donation avoidance. Women were more concerned about adverse physical consequences, and nondonors expressed higher levels of groundless donation-related fears.

CONCLUSION

Recruiters should consider the sex and donation experience of targets when they develop recruitment and retention strategies. Education programs aimed at overcoming fears and heightening awareness of need are recommended, as are operational improvements aimed at reducing barriers posed by time and inconvenience.

Two-dimensional crystallization of herpes simplex virus type 1 single-stranded DNA-binding protein, ICP8, on a lipid monolayer

Herpes simplex virus type 1 single-stranded DNA-binding protein (ICP8) has been crystallized on a positively charged lipid monolayer. The crystals belong to the planar group p2 with a=39 nm, b=23.2 nm and gamma=87.2 degrees. The projected map of ICP8 crystals calculated at a resolution of 3.9 nm shows four ICP8 monomers per unit cell with the crystals formed by a parallel arrangement of 16.2 nm helical ICP8 filaments. This novel filamentous form has not been reported before. The ICP8 monomers show different appearances in projection, suggesting that they may adopt different orientations, probably reflecting the strong intermolecular and lipid-filament interactions in the crystal. When the 23 nm diameter filaments formed by ICP8 in solution at low temperature in the presence of magnesium were generated and then layered on the phospholipid monolayer, highly ordered arrays of an 8.5 nm filament with a shallow 31.2 nm pitch were observed and reconstruction revealed a double-helical structure.

A 3-D reconstruction of smooth muscle alpha-actinin by CryoEm reveals two different conformations at the actin-binding region

Cryoelectron microscopy was used to obtain a 3-D image at 2.0 nm resolution of 2-D arrays of smooth muscle alpha-actinin. The reconstruction reveals a well-resolved long central domain with 90 degrees of left-handed twist and near 2-fold symmetry. However, the molecular ends which contain the actin binding and calmodulin-like domains, have different structures oriented approximately 90 degrees to each other. Atomic structures for the alpha-actinin domains were built by homology modeling and assembled into an atomic model. Model building suggests that in the 2-D arrays, the two calponin homology domains that comprise the actin-binding domain have a closed conformation at one end and an open conformation at the other end due to domain swapping. The open and closed conformations of the actin-binding domain suggests flexibility that may underlie Ca2+ regulation. The approximately 90 degrees orientation difference at the molecular ends may underlie alpha-actinin's ability to crosslink actin filaments in nearly any orientation.

The immune response of regional lymph nodes during the early stages of Fasciola hepatica infection in cattle

In this study we examined regional immune responses to Fasciola hepatica infection in the natural ruminant host. Naïve cattle and those pre-exposed to a drug-abbreviated infection were subsequently challenged and lymph nodes extracted at slaughter. In vitro proliferation and cytokine production by mononuclear cells isolated from hepatic and mesenteric lymph nodes were measured after culture with whole fluke antigen (WFA). Hepatic lymph node cells had a significantly greater response to parasite antigen than mesenteric lymph node cells (P < 0.02), although there was no difference in the magnitude of the proliferative response between naïve and pre-exposed challenged cattle. Mononuclear cells from hepatic lymph nodes produced interferon gamma, interleukin 2 and interleukin 4 after culture with parasite antigen, indicative of a mixed, T helper type 0, response. Comparison of the hepatic node response to a variety of F. hepatica antigens showed that proliferation was lower after culture with cathepsin-L, than with a high molecular weight fraction, WFA or excretory-secretory antigen. Cell culture supernatant fluid from unstimulated hepatic lymph node cells showed an IgG1 response to antigens of 48, 52-70, 82, 96 and 120-190 kDa on Western blot in pre-exposed, but not naïve, challenged animals.

Pre-exposure of cattle to drug-abbreviated Fasciola hepatica infections: the effect upon subsequent challenge infection and the early immune response

In this study we examined whether juvenile liver flukes are capable of stimulating protective immune responses in cattle. Four experimental groups of cattle were studied as follows: group A, a positive control, received a primary infection on day 0 and a secondary infection 28 days later; group B also received two infections but the primary infection was terminated by drug treatment on day 5; group C, received infections on days 0, 5 and 10 which were terminated by drug treatments on days 1, 6 and 11 and then a secondary infection on day 28; group D received an infection only on day 28. Juvenile flukes appear to induce protective responses because: (a) group B animals had significantly lower levels of gamma-GT (P<0.05) than group D; (b) both groups B and C exhibited lower parenchymal phase GLDH levels (P=0.006 and 0.041, respectively); and (c) both groups B and C had lower secondary phase eosinophilia (P=0.002 and 0.02, respectively) than those in group D. Sera taken from groups A-C contained antibodies reacting to a variety of proteins in adult fluke somatic antigen and excretory-secretory preparations, particularly to proteins of 52-60, 68-72 and 82-96 kDa. After secondary challenge the antibody responses of group A to these proteins declined while reactivity to proteins of 28-30 kDa increased. Antibody responses to the 28-30 kDa proteins were not detected in groups B-D until 3 weeks later than those observed in group A. Antibody responses to Fasciola hepatica cathepsin L proteases, which are known to induce protection, were monophasic, of the IgG1 isotype only and were not observed prior to secondary challenge in any of the four groups. In contrast, the response to another protective antigen fraction, a high molecular sized haem protein, was of a mixed IgG1/IgG2 nature and was detected within 14 days of primary infection. However, no significant difference in antibody titres to either protein preparation was observed after the secondary infection when groups B and C were compared to group D.

Analysis of pooled data from the randomised controlled trials of endarterectomy for symptomatic carotid stenosis

BACKGROUND

Endarterectomy reduces risk of stroke in certain patients with recently symptomatic internal carotid stenosis. However, investigators have made different recommendations about the degree of stenosis above which surgery is effective, partly because of differences between trials in the methods of measurement of stenosis. To accurately assess the overall effect of surgery, and to increase power for secondary analyses, we pooled trial data and reassessed carotid angiograms.

METHODS

We pooled data from the European Carotid Surgery Trial (ECST), North American Symptomatic Carotid Endarterectomy Trial, and Veterans Affairs trial 309 from the original electronic data files. Outcome events were re-defined, if necessary, to achieve comparability. Pre-randomisation carotid angiograms from ECST were re-measured by the method used in the other two trials.

RESULTS

Risks of main outcomes in both treatment groups and effects of surgery did not differ between trials. Data for 6092 patients, with 35000 patient-years of follow-up, were therefore pooled. Surgery increased the 5-year risk of ipsilateral ischaemic stroke in patients with less than 30% stenosis (n=1746, absolute risk reduction -2.2%, p=0.05), had no effect in patients with 30-49% stenosis (1429, 3.2%, p=0.6), was of marginal benefit in those with 50-69% stenosis (1549, 4.6%, p=0.04), and was highly beneficial in those with 70% stenosis or greater without near-occlusion (1095, 16.0%, p<0.001). There was a trend towards benefit from surgery in patients with near-occlusion at 2 years' follow-up (262, 5.6%, p=0.19), but no benefit at 5 years (-1.7%, p=0.9).

INTERPRETATION

Re-analysis of the trials with the same measurements and definitions yielded highly consistent results. Surgery is of some benefit for patients with 50-69% symptomatic stenosis, and highly beneficial for those with 70% symptomatic stenosis or greater but without near-occlusion. Benefit in patients with carotid near-occlusion is marginal in the short-term and uncertain in the long-term.

Changes in properties of adenosine transporters in Trypanosoma evansi and modes of selection of resistance to the melaminophenyl arsenical drug, Mel Cy

Resistance to arsenical drugs in trypanosomes has been linked to changes in adenosine uptake. The transport of melaminophenyl arsenicals into Trypanosoma brucei was shown to be mediated by an unusual adenosine nucleoside transporter, P2 (Carter and Fairlamb, 1993), and the loss of this transporter is associated with resistance to melaminophenyl arsenicals in these parasites. To further understand the mechanisms of arsenical resistance, we generated several lines of Mel Cy-resistant T. evansi from a drug-sensitive isolate using both in vivo and in vitro selection methods. Uptake of the melaminophenyl arsenical, Mel Cy on the P2 transporter was studied in the drug-sensitive as well as Mel Cy-resistant parasites, by means of inhibition of Mel Cy-induced lysis of trypanosomes, in an in vitro lysis assay. Adenosine uptake was also investigated using competition inhibition assays. Our study shows that T. evansi, TREU 1840, possesses the P1/P2 adenosine transport system as reported in T. brucei and T. equiperdum. However, in T. evansi, the P2 transporter is the larger transport process instead of the P1. The P2 transporter in T. evansi mediated the uptake of Mel Cy in the drug-sensitive parasites. The P2 was retained in all the arsenical-resistant T. evansi lines studied. However, the activity of the transporter was reduced to different extents in the different-resistant lines. The residual P2 activity related well to the levels of drug resistance in each line, suggesting that P2 activity could be an important marker for arsenical resistance. Furthermore, important differences were observed between the in vivo- and the in vitro-selected arsenical-resistant parasites suggesting that there may be differences in resistance phenotypes selected on the field.

Gravidity-dependent production of antibodies that inhibit binding of Plasmodium falciparum-infected erythrocytes to placental chondroitin sulfate proteoglycan during pregnancy

During pregnancy, Plasmodium falciparum-infected erythrocytes sequester in the placenta by adhering to chondroitin 4-sulfate, creating a risk factor for both the mother and the fetus. The primigravidae are at higher risk for placental malaria than the multigravidae. This difference in susceptibility has been attributed to the lack of antibodies that block the adhesion of infected erythrocytes to placental chondroitin 4-sulfate in primigravid women. However, recent results show that many primigravidae at term have antibody levels similar to those of multigravidae, and thus the significance of antiadhesion antibodies in providing protection against malaria during pregnancy remains unclear. In this study, we analyzed plasma samples from women of various gravidities at different gestational stages for antiadhesion antibodies. The majority of women, regardless of gravidity, had similar levels of antibodies at term. Most primigravidae had low levels of or no antiadhesion antibodies prior to ~20 weeks of pregnancy and then produced antibodies. Multigravidae also lacked antibodies until ~12 weeks of pregnancy, but thereafter they efficiently produced antibodies. In pregnant women who had placental infection at term, higher levels of antiadhesion antibodies correlated with lower levels of placental parasitemia. The difference in kinetics of antibody production between primigravidae and multigravidae correlated with the prevalence of malaria in these groups, suggesting that antibodies are produced during pregnancy in response to placental infection. The early onset of efficient antibody response in multigravidae and the delayed production to antibodies in primigravidae appear to account for the gravidity-dependent differential susceptibilities of pregnant women to placental malaria.

Acquisition and decay of antibodies to pregnancy-associated variant antigens on the surface of Plasmodium falciparum-infected erythrocytes that protect against placental parasitemia

Otherwise clinically immune women in areas endemic for malaria are highly susceptible to Plasmodium falciparum malaria during their first pregnancy. Pregnancy-associated malaria (PAM) is characterized by placental accumulation of infected erythrocytes that adhere to chondroitin sulfate A (CSA). Susceptibility to PAM decreases with increasing parity, apparently due to acquisition of antibodies directed against the variant surface antigens (VSAs) that mediate the adhesion to CSA (VSA(CSA)). This study found that levels of VSA(CSA)-specific antibodies depend on endemicity, that anti-VSA(CSA) IgG is acquired during gestation week 20, and that plasma levels of the antibodies decline during the postpartum period. There is evidence that VSA(CSA)-specific antibodies are linked to placental infection and that high antibody levels contribute to the control of placental infection by inhibiting parasite adhesion to CSA. Data suggest that VSA(CSA) is a target for vaccination against PAM.

The three-dimensional structure of alpha-actinin obtained by cryoelectron microscopy suggests a model for Ca(2+)-dependent actin binding

The three-dimensional structure of alpha-actinin from rabbit skeletal muscle was determined by cryoelectron microscopy in combination with homology modeling of the separate domain structures based on results previously determined by X-ray crystallography and nuclear magnetic resonance spectroscopy. alpha-Actinin was induced to form two-dimensional arrays on a positively charged lipid monolayer and micrographs were collected from unstained, frozen hydrated specimens at tilt angles from 0 degrees to 60 degrees. Interpretation of the 15 A-resolution three-dimensional structure was done by manually docking homologous models of the three key domains, actin-binding, three-helix motif and the C-terminal calmodulin-like domains. The initial model was refined quantitatively to improve its fit to the experimental reconstruction. The molecular model of alpha-actinin provides the first view of the overall structure of a complete actin cross-linking protein. The structure is characterized by close proximity of the C-terminal, calmodulin-like domain to the linker between the two calponin-homology domains that comprise the actin-binding domain. This location suggests a hypothesis to explain the involvement of the C-terminal domain in Ca(2+)-dependent actin binding of non-muscle isoforms.

Polymorphism of bovine major histocompatibility complex (MHC) class I genes revealed by polymerase chain reaction (PCR) and restriction enzyme analysis

The polymorphic exon 2-exon 3 region of bovine major histocompatibility complex (MHC) class I genes was amplified by polymerase chain reaction (PCR) from genomic DNA samples with characterized class I polymorphism. The primers for amplification were designed in conserved regions at the borders of exons 2 and 3, based on all available cDNA sequences. The primers should, therefore, amplify most expressed class I genes, but may also amplify non-expressed class I genes. The PCR amplified class I gene fragments of 700 bp were characterized on the basis of restriction fragment length polymorphism (RFLP). The PCR-RFLP analysis of class I genes showed that the bands in each digestion could be classified as non-polymorphic, as shared between several bovine lymphocyte antigen (BoLA)-A types, or as specific to a single BoLA-A type. The same primers were then used for amplification of class I gene fragments from eight Sahiwal animals, a breed which originated in the Indian subcontinent. These studies showed that BoLA class I PCR-RFLP could be used to study class I polymorphism in family groups.

The epidemiology of Plasmodium falciparum malaria in two Cameroonian villages: Simbok and Etoa

In support of ongoing immunologic studies on immunity to Plasmodium falciparum, demographic, entomologic, parasitologic, and clinical studies were conducted in two Cameroonian villages located 3 km apart. Simbok (population = 907) has pools of water present year round that provide breeding sites for Anopheles gambiae, whereas Etoa (population = 485) has swampy areas that dry up annually in which A. funestus breed. Results showed that individuals in Simbok receive an estimated 1.9 and 1.2 infectious bites per night in the wet and dry season, respectively, whereas individuals in Etoa receive 2.4 and 0.4 infectious bites per night, respectively. Although transmission patterns differ, the rate of acquisition of immunity to malaria appears to be similar in both villages. A prevalence of 50-75% was found in children < 10 years old, variable levels in children 11-15 years old, and 31% in adults. Thus, as reported in other parts of Africa, individuals exposed to continuous transmission of P. falciparum slowly acquired significant, but not complete, immunity.

Analysis of genes expressed at the infective larval stage validates utility of Litomosoides sigmodontis as a murine model for filarial vaccine development

We used an expressed sequence tag approach to analyze genes expressed by the infective larvae of the rodent filarial parasite Litomosoides sigmodontis. One hundred fifty two new genes were identified, including several proposed as vaccine candidates in studies with human filarial parasites. Our findings have important implications for the use of L. sigmodontis as a model for filarial infection.

Clinical practice guidelines: the need for improved implementation strategies

Although substantial resources have been spent developing clinical practice guidelines, little effort has been made to evaluate the effectiveness of their implementation. In the absence of effective implementation, CPGs will have minimal impact on changing behaviours. This article critically reviews the literature and identifies a number of reasons why these guidelines have not been implemented successfully. If CPGs are to have a positive impact upon clinical behaviour and practice, then more aggressive and positive implementation strategies are indicated. Five key ingredients for successful implementation of quality CPGs are identified and recommended.

Interleukin-5 is essential for vaccine-mediated immunity but not innate resistance to a filarial parasite

The study of protective immune mechanisms effective against filarial nematodes has been hampered by the inability of these important human pathogens to infect laboratory mice. Recently, Litomosoides sigmodontis, a natural parasite of rats, has been developed as a valuable model for the study of filarial infection. BALB/c mice are fully susceptible to infection with L. sigmodontis third-stage larvae and develop patent infection. In contrast, mice on the C57BL background are resistant, and parasites undergo only a single molt and do not mature to adulthood. We used interleukin-5 (IL-5)-deficient mice on the C57BL/6 background to address the role of IL-5 and eosinophils in the innate resistance of C57BL/6 mice. We found no differences in parasite survival between IL-5-deficient and C57BL/6 mice. However, when these mice were used for the analysis of vaccine-mediated immunity, a critical role for IL-5 was elucidated. Mice genetically deficient in IL-5 were unable to generate a protective immune response when vaccinated with irradiated larvae, whereas C57BL/6 mice were fully protected from challenge infection. These studies help to clarify the highly controversial role of eosinophils in filarial infection.